When an internal combustion engine is operating in a lean burn mode, there is always an excess of oxygen present in the exhaust system. As a result, oxides of nitrogen (NOx) present in the exhaust gases while the engine is operating in lean burn mode cannot be reduced by a three-way catalytic converter. To avoid such NOx gases from being discharged into the ambient atmosphere, it has been proposed to store them temporarily in a trap and to release them into the exhaust stream at a later stage when the engine is operated with a rich or at least stoichiometric mixture.
NOx traps can be formed as modified three-way catalytic converters in which the matrix has additional NOx storage sites consisting of base metal oxides. In the presence of surplus oxygen and NOx gases, the base metal oxides are converted to nitrates thereby trapping the NOx gases. In a reducing atmosphere on the other hand, the metal nitrates decompose releasing the NOx back into the exhaust stream but at this time the functioning three-way catalyst will reduce the NOx gases to nitrogen that can safely be discharged into the atmosphere.
A problem encountered with such traps is that the base metal oxides will also react with oxides of sulphur to produce sulphates which are more stable compounds than nitrates and do not normally decompose when the engine is again run in a rich or stoichiometric mode. As a result, unless further steps are taken, the trap is poisoned by sulphur which, at least in Europe, is usually present in the fuel.
It has been found that the damage done to a trap by sulphur in the fuel is reversible in the presence of a reducing atmosphere if the trap is heated to above 650.degree. C.